This invention relates to a method of measuring the velocity and/or length of moving objects in accordance with the Doppler principle.
The invention also relates to an arrangement for performing the method.
For the contactless measurement of the velocity and/or the length of moving objects the Doppler principle can be utilized. The frequency shift imparted to an electromagnetic wave after reflection from a moving object is a measure of the velocity v of the object. The length S of the object (or the distance travelled by a point on the object) can be derived from the velocity by integration of the velocity over time t: S=.intg.v dt.
Such a method and arrangement are disclosed in the published European Patent Application No. 21048. According to this method two pairs of mutually non-interfering partial beams are generated from a light beam originating from a laser, which beams are projected onto the object to be measured. With each pair of beams the velocity in one dimension is determined by measuring the Doppler shift to which the relevant pair of beams has been subjected. (Strictly speaking, the velocity information is determined from the difference between the Doppler-shifts of the first and second beams of a pair of beams). The velocity range of the objects to be measured determines the width of the frequency band in which the Doppler frequency shifts are located. If the velocity range in which a measurement is to be made is large, then the signal processing unit with which the frequency shift is determined must have a large bandwidth. Owing to variations in reflection or scattering properties of the moving object, interference components are produced in the signal reflected from the object. When the surface of the object has a fine structure, these frequency components may be included in the frequency band of the actual signal, causing the accuracy of the measurement to be influenced adversely. In order to obviate this disadvantage to some extent, it has already been proposed to limit the bandwidth of the signal processing unit by providing a narrow-band band pass filter at the input thereof. This makes it possible to measure only a limited velocity range. Therefore, in order to make it possible to perform measurements in a broad overall velocity range, it has already been proposed to use a filter unit formed by a plurality of bandpass filters having mutually different centre frequencies. By activating that filter which is nearest to the frequency shift produced, narrow-band filtering is then still possible.
In methods other than the above-mentioned proposed methods, use is made of a frequency tracking system (which is commonly referred to as a "tracker"). In those systems the signal processing unit is electronically adjusted so that the very wide frequency variations occurring in the detector signal are tracked.
However, these two systems have the disadvantage that the signal processing unit is complicated and the accuracy for some applications nevertheless leaves much to be desired.